1. Field of the Invention
This invention generally relates to fastening devices and, more particularly, to a device and system for recessing a fastener on a printed wire board (PWB).
2. Description of the Related Art
FIGS. 1A and 1B are drawings depicting fasteners on a PWB (prior art). FIGS. 1A and 1B are not drawn to scale. Fasteners, such as screws, are typically used to install a PWB in a structure, for example, the chassis of a wireless communications device. As shown in FIG. 1A, fastener heads can protrude above the surface of a PWB. Such protrusions can cause potential interference problems with the installation and layout of components to be connected to the PWB or with other PWBs or components near the PWB. As shown in FIG. 1B, a countersink hole can be provided at the location of the fastener on the PWB to recess the fastener head and prevent fasteners from protruding above the surface of the PWB. Typically, countersinking is possible with PWBs having a thickness greater than approximately one millimeter.
Due to advances in PWB fabrication techniques, PWBs with thicknesses of less than approximately one millimeter are now available. The use of these thinner PWBs is desirable, since these PWBs, by virtue of their own reduced size and weight, reduce the size and weight of devices into which the PWBs are installed. Unfortunately, countersinking is typically not possible using conventional designs, layouts, and materials for PWBs having a thickness less than approximately one millimeter. Therefore, the reduced size and weight advantages of the thinner PWBs are offset by potential interference problems related to protruding fasteners.
For thicker PWBs, there are still disadvantages to countersinking. Contersinking holes in a PWB involves extra process steps that typically must be performed in small batches, or individually, eliminating the possibility of processing multiple boards in one operation, as is possible if only through-holes are required. For example, when through-holes are used, a group of PWBs having the same fastener hole locations can be stacked together, aligned, and drilled as a group, reducing the time and cost to produce holes necessary for fastening the PWBs.
Conventionally, fasteners are allowed to protrude above the surface of PWBs with thicknesses less than approximately one millimeter. As a result, users and manufacturers of devices containing these PWBs have suffered from the consequences of protruding fasteners.
It would be advantageous if a fastener for a PWB with a thickness of less than approximately one millimeter could be fully or partially recessed below the surface of the PWB. It also would be advantageous if countersinking could be eliminated for PWBs with thicknesses greater than approximately one millimeter, allowing the use of standard through-holes for fasteners.
The present invention addresses the problem of interferences associated with the use of fasteners on a printed wire board (PWB). The invention also addresses the use of countersinking operations that preclude more efficient methods for drilling fastener holes in PWBs. The invention addresses these problems by attaching a recessed fastener adapter to a PWB at a standard (non-countersunk) through-hole in the PWB. A fastener is inserted in the adapter and the shaft of the fastener passes through the adapter (allowing the fastener to be used to attach the PWB to a structure). The adapter secures and recesses the head of the fastener fully or partially below the top surface of the PWB. The adapter itself either does not cause interferences on the PWB, or causes interferences, the extent of which are less than the interferences that would be caused by not using the adapter or are acceptable to obtain the benefit of eliminating counter-sink holes.
Accordingly, a system is provided for recessing a fastener on a PWB. The system includes a PWB having a fastener hole, a top surface, and a bottom surface. The system further comprises a recessed fastener adapter that includes a board interface for interfacing the adapter with the PWB at the fastener hole and a fastener head engaging surface for accepting and recessing a fastener head. The fastener head engaging surface recesses a fastener head at least partially below the PWB top surface.
The board interface is generally in the shape of a tube. The tube is not necessarily cylindrical. That is, the walls of the tube may be circular, curved or straight in a cross-section perpendicular to the long axis of the tube. The tube has an exterior surface that is connected to the PWB at the fastener hole. The fastener head engaging surface is formed by a closed end on the tube having an opening or bore. A fastener is inserted into the other (open) end of the tube, the shaft of the fastener passes through the opening in the closed end, and the head of the fastener rests against the closed end.
In one version of the adapter, a flange extends radially outward from the exterior surface of the tube. The flange is located at or near the open end of the tube. The flange has a disc-like shape with an upper surface (facing away from the closed end of the tube). To install the adapter, the upper surface is placed against the bottom surface of a PWB at a fastener hole. The shape of the flange outline and the configuration of the flange upper surface can vary as long as the upper surface properly seats against the PWB bottom surface. The opening in the closed end of the tube and the fastener hole are aligned so that a fastener can be inserted from the top side of the PWB through the fastener hole. The fastener shaft passes through the opening in the closed end of the tube and the fastener head rests against the inside surface of the closed end of the tube. The head of the fastener is either fully or partially contained in the tube and is, therefore, either fully or partially recessed below the top surface of the PWB. The degree of the recess can be adjusted so that the fastener head does not cause interferences on the PWB, so that fastener head interferences are less than would be caused by the fastener head without the adapter, or so that fastener head interferences are acceptable to obtain the benefit of eliminating counter-sink holes.
In one aspect of the present invention, a solder bond is used to connect the tube to the PWB, but if the bond is not strong enough, an adhesive material is used is connect the tube to the PWB. Accordingly, the surfaces of the tube in contact with the PWB (in this case, the upper surface of the flange) are made of materials that accept an adhesive material or are suitable for solder bonding.
To use a recessed fastener adapter to secure a PWB to the structure of a device such as the chassis of a wireless communications device, the adapter is installed on the PWB and a fastener is inserted into and through the adapter, as described above. The fastener, in turn, is connected to the structure of the device. The connection can take a number of forms including screw threads engaging the structure or a rivet expanding in the structure. As the fastener is connected to the structure, the fastener head advances toward the structure, contacts the recessed fastener adapter, and pulls the recessed fastener adapter toward the structure. The pressure from the pull is transferred to the PWB, securing the PWB to the device structure in the desired position.
The use of the present invention system enables fasteners to be recessed on PWBs with thicknesses less than approximately one millimeter. For PWBs with thicknesses greater than approximately one millimeter, the use of the present invention system eliminates countersinking at PWB fastener holes and allows the use of through-holes which can be drilled in a more efficient manner than countersink holes. Additional details of the above-mentioned system and a device for recessing a fastener on a PWB are presented below.